Astrophysics of Galaxies (astro-ph.GA)

Abstract: This work is the seventh study in a series dedicated to investigating degeneracies of simultaneous age, mass, extinction, and metallicity determinations of partially resolved or unresolved star clusters with Hubble Space Telescope broadband aperture photometry. In the sixth work (hereafter, Paper I), it was demonstrated that the adaptive aperture photometry, performed to avoid the majority of the projected foreground and background stars falling within the apertures, gives more consistent colour indices for star clusters. In this study, we aim to supplement the homogeneous multi-colour aperture photometry results published in Paper~I and provide a complete M31 Panchromatic Hubble Andromeda Treasury (PHAT) survey star cluster photometry catalogue for further analysis. Following Paper I, we used a two-aperture approach for photometry. The first aperture is the standard one used to measure total cluster fluxes. The second (smaller) aperture is introduced to avoid the bright foreground and background stars projecting onto the clusters. We selected the radii of smaller apertures to be larger than the half-light radii of the clusters. We present the second part of the star cluster aperture photometry catalogues for a sample of 1477 star clusters from the M31 PHAT survey not covered in Paper I. Compared to the M31 PHAT star cluster aperture photometry catalogue published by Johnson et al., adjustments were made to the cluster centre coordinates, aperture sizes, and sky background levels.

Abstract: Galaxy rotation curve (RC) fitting is an important technique which allows the placement of constraints on different kinds of dark matter (DM) halo models. In the case of non-phenomenological DM profiles with no analytic expressions, the art of finding RC best-fits including the full baryonic $+$ DM free parameters can be difficult and time-consuming. In the present work, we use a gradient descent method used in the backpropagation process of training a neural network, to fit the so-called Grand Rotation Curve of the Milky Way (MW) ranging from $\sim$1 pc all the way to $\sim$$10^5$ pc. We model the mass distribution of our Galaxy including a bulge (inner $+$ main), a disk, and a fermionic dark matter (DM) halo known as the Ruffini-Arg\"uelles-Rueda (RAR) model. This is a semi-analytical model built from first-principle physics such as (quantum) statistical mechanics and thermodynamics, whose more general density profile has a dense core -- diluted halo morphology with no analytic expression. As shown recently and further verified here, the dark and compact fermion-core can work as an alternative to the central black hole in SgrA* when including data at milliparsec scales from the S-cluster stars. Thus, we show the ability of this state-of-the-art machine learning tool in providing the best-fit parameters to the overall MW RC in the $10^{-2}$--$10^5$ pc range, in a few hours of CPU time.

Abstract: Galactic archaeology represents a multidisciplinary approach aimed at unraveling the intricate history of the Milky Way galaxy through the study of its stellar populations. This abstract delves into the significance of galactic archaeology as a vital tool for understanding the formation and evolution of the Milky Way. By examining the distribution, kinematics, chemical compositions, and ages of stars within the galaxy, researchers gain insights into the dynamic processes that have shaped its structure over billions of years. Stellar populations serve as invaluable relics of past epochs, preserving clues about the conditions prevalent during their formation. The utilization of spectroscopic and photometric data has enabled the classification and analysis of stars, allowing astronomers to discern distinct populations and unveil their origin stories. Through these studies, the emergence of the Milky Way's various components, such as the thin and thick disk, halo, and bulge, becomes discernible.

Abstract: We analyse a sample of 21 `bare' Seyfert~1 Active Galactic Nuclei (AGNs), a sub-class of Seyfert~1s, with intrinsic absorption $\mathrm{N_{H}} \sim 10^{20}~ \mathrm{cm}^{-2}$, in the local universe (z $<$ 0.2) using {\it XMM-Newton} and {\it Swift}/XRT observations. The luminosities of the primary continuum, the X-ray emission in the 3 to 10 keV energy range and the soft-excess, the excess emission that appears above the low-energy extrapolation of the power-law fit of 3 to 10 keV X-ray spectra, are calculated. Our spectral analysis reveals that the long-term intrinsic luminosities of the soft-excess and the primary continuum are tightly correlated $(L_{PC}\propto L_{SE}^{1.1\pm0.04})$. We also found that the luminosities are correlated for each source. This result suggests that both the primary continuum and soft excess emissions exhibit a dependency on the accretion rate in a similar way.

Abstract: High-redshift low-mass galaxies are believed to be the building blocks of present-day galaxies. Their sheer prevalence, compared to their more massive counterparts, makes them the most representative examples of the first generation of galaxies. Furthermore, they likely played a pivotal role in cosmic reionization between redshifts of $z=9$ and $z=6$. However, this population has continued to elude comprehensive spectroscopic studies. As a consequence, their role in comic reionization has remained unclear, owing to the uncertainties surrounding their photometric redshifts and the lack of constraints on both their ionizing photon production and escape fraction. Here we report the first spectroscopic analysis of 8 ultra-faint galaxies during the epoch of reionization with absolute magnitudes between M$_{\rm UV} \sim -17$ to $-15$ mag (down to 0.005 $L^{\star}$). The combination of ultra-deep NIRSpec (Near-Infrared Spectrograph) observations and the strong gravitational lensing of Abell~2744 allows us to explore an uncharted territory of early galaxy formation. Remarkably, some of these galaxies have stellar masses as low as $7.6_{-1.4}^{+1.7} \times 10^{5}$ \msol, comparable to dwarf galaxies in the local group, and extremely low metallicities of only 1 to 6\% $Z_{\odot}$. This sample allows us to derive the first combined spectroscopic constraints on both the prevalence of faint galaxies and their ionizing properties during the Universe's first billion years. We find that faint galaxies have an ionizing efficiency of log($\xi_{\rm ion}$/ Hz erg$^{-1}$)=$25.8\pm 0.05$, about a factor of 4 larger than canonical values. This means that the total ionizing photon budget produced by galaxies exceeds the reionization threshold, even for modest values of $f_{\rm esc}$ =5%. These findings provide robust evidence that faint galaxies were the main drivers of cosmic reionization at $z\sim7$.